Snow Leopard had a default smart folder called "Today". This folder was awesome! All "Open" dialog boxes could easily display all of the files you've worked on "Today" (which are probably the ones you want 'right now').

Phew! I decided to do more digging on my own. I took a look at a computer still using Snow Leopard and right-clicked on its "Today" folder to reveal its path: /System/Library/CoreServices/Finder.app/Contents/Resources/CannedSearches It turns out this directory still exists in Lion! So go to Finder, and use the Go To Folder command (cmd-shift-g), and enter that path above. The "canned searches" are all still there. Drag and drop them to your sidebar. Nirvana.
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avdempseyAug 6 '11 at 1:50

Open Finder and clicked on your User directory, then enter a dummy search term, e.g. 'foo' to start the process.

Click the '+' button next to save. This will add a term to the search.

Hold down the alt/option key and the + button should become a .... Click the ... button twice to add two groups of parameters.

Now that you have two parameter groups, make the first group None of the following are true and add the things you want to exclude, such as Kind is Folder.

Make the second group Any of the following are true and add your three date options:

Last opened date is within last 1 days

Last modified date is within last 1 days

Created date is within last 1 days.

Remove your dummy search term and the first search term in the list.

Save the search and show it in your sidebar:

Now you can access the Today saved search folder from your sidebar:

Notes:

I found that the "Today" Search in /System/Library/CoreServices/Finder.app/Contents/Resources/CannedSearches doesn't show files that were created today, but not subsequently opened, so it didn't meet my needs.

@nathan's answer describes the process well (and was the basis for this answer), but I am adding this answer with screen-shots to help the more visually inclined.

Like others, the existing “Today” canned search doesn’t quite fit all my needs. So, I tried the answer first suggested by @nathan and illustrated by @adam-franco, but found that there was still a piece missing: items downloaded in the last day did not get included in the search results.

The solution is simple. Just add “Date added” (under “Other…” in the search parameter dropdown) to the second parameter group, like so: